This invention relates to a hydraulic ram having a plurality of magnets embedded in its piston rod and sensing means associated with its cylinder. Hydraulic rams of this type are particularly useful as advance rams for longwall mineral mining installations.
A known type of hydraulic ram has a plurality of permanent magnets embedded in its piston rod, and a sensor fixed to its cylinder. The sensor incorporates an induction coil or a magnetic switch, and so senses the magnets as it passes over them during extension or retraction of the ram. Such a sensor will emit a control signal whenever a magnet is sensed, and these control signals can be used to measure a working stroke of the ram. Hydraulic rams of this type are used in particular for control purposes in longwall mineral mining installations. They serve, for example, for effecting a synchronisation (or follow-up control) of hydraulic rams such as, for example, the advance rams of mine roof support bar extensions of mine roof support units. (See U.S. Pat. No. 4,307,981).
When hydraulic rams of this type are used as advance rams for underground conveying and winning installations, the permanent magnets can also be used for measuring the distances through which the rams are extended or retracted, and therefore for measuring the advance of the installation. In this system, a row of magnets can be arranged on the piston rod, so that, when the advance stroke of the ram takes place, the magnets move in succession past the sensor (which is arranged at the end of the cylinder). In so doing, they provide electrical control signals which can be counted by an electronic counter which provides a measure of the advanced distance.
In a known hydraulic ram of this type, the permanent magnets are housed in metal bushes, each of which is held by a force-fit in a respective radial bore formed in the piston rod of the ram. The sensor, which is positioned on the end of the cylinder of the ram in alignment with the row of magnets, is a probe or switch, for example a magnetic or proximity switch, which is actuated as the permanent magnets move past it.
It has been proposed to arrange two or more rows of magnets on the piston rod of a hydraulic ram in such a way that the permanent magnets of one row are axially offset from those of the other row(s). (See DE-OS No. 3 137 951). In this arrangement, a separate probe or switch is provided for each row of magnets. Hydraulic rams of this type are particularly useful as hydraulic advance rams in underground mineral mining installations. The arrangement of the permanent magnets in several rows not only offers the possibility of spacing the magnets more widely from each other, and thus of suppressing reciprocal magnetic effects, but also renders it possible to preclude incorrect measurements from so-called "breathing" movements, that is to say movements resulting from a longwall conveyor being forced backwards towards the goaf side of a working by a passing winning machine such as a plough.
The aim of the invention is to provide an improved hydraulic ram of the type having a plurality of magnets embedded in its piston rod and sensing means associated with its cylinder.